The present invention relates To an air filter for an internal combustion engine, especially for a two-stroke engine for hand-held working tools such as motor chainsaws, a cutter or trimmer etc. The air filter is comprised of a housing with housing walls formed by filter elements through which the combustion air is sucked into the interior chamber of the housing. A suction opening is provided in one of the housing walls for connecting the air filter to the suction channel of the combustion engine.
In internal combustion engines, especially in two-stroke combustion engines, gas pulsations occur within the suction channel which, due to the design and operation of the combustion engine, cannot be avoided. Due to these pulsations, a portion of the combustion air enriched with fuel, and in the case of two-stroke combustion engines also with oil, flows counter to the suction direction back into the air filter. The fuel and/or the oil are deposited on the filter surfaces so that the wet filter surface is soiled and will subsequently be clogged. Thus, frequent filter changes are required.
In order to minimize the escape of suction air flow into the air filter, it is suggested according to U.S. Pat. No. 4,600,418 to embody the surface of the air filter facing the suction channel as a rebound wall with a curvature such that the pulsating gas flow is deflected in the direction toward the suction channel in the direction of flow of suction air. With such a measure it is indeed possible to reduce the danger of wetting and soiling of the air filter. However, the resulting service life of such air filters cannot be increased to the extent desired.
It is therefore an object of the present invention to provide an air filter of the aforementioned kind with which a wetting of the filter element by backwashing of the fuel/air mixture is prevented.